Hydrogen purifiers are devices that separate hydrogen from hydrogen-rich gaseous mixtures, providing a pure stream of hydrogen for a variety of uses. Typical applications include supplying hydrogen for fuel cells from reformed gases, purifying commercial grade hydrogen to provide for ultra-pure hydrogen used for semiconductor processing, supplying hydrogen for the food industry, purifying hydrogen from an electrolysis stream for laboratory uses, and many other industrial applications.
Typically hydrogen purifiers utilize a thin, hydrogen-permeable metal membrane to effectively separate hydrogen from a gaseous mixture containing hydrogen. While there are a variety of alloys, the most commonly used alloys are Pd77Ag23 and Pd60Cu40. These alloys are rolled into foils that are approximately 25 μm thick, and are then incorporated into a hydrogen purifier. Thinner foils may be used, but they are more fragile and prone to pinhole leaks, which degrades the purity of the supplied hydrogen.
A purifier may ideally operate between about 250° C. and 700° C. depending on the operating condition requirements, the material construction constraints of the purifier, and the gases introduced. For example, in a methanol steam reformer, the hydrogen rich gas produced by the reforming reaction will require a purifier operational temperature of at least about 300° C., in order to reduce the deleterious effects of carbon monoxide at the membrane surface. In other cases the reformer may use a different fuel, such as natural gas, in which the steam reforming temperature will be much higher, such as 550° C.-700° C. These different constraints will require different materials; for operation at 300-400° C. Pd60Cu40 will work well, but above about 400° C. Pd77Ag23 may be preferred due to its superior durability at higher temperatures. In either case, steam reforming with subsequent hydrogen separation utilizing a purifier will typically require a gas pressure in the range of 5-20 atmospheres on the high pressure side of the membrane for effective operation of the purifier. The permeate side pressure will vary depending on the output flux according to Sievart's law. Further, in some cases the temperature of the reformed gases must be reduced prior to their introduction to the purifier; it is generally desirable to maintain purifiers below 450° C. to prevent unwanted intermetallic diffusion between the membrane and the membrane support or seals at the perimeter. Conversely, purifiers will generally be operated above 300° C. to reduce the effects of carbon monoxide coverage (blockage) at the palladium membrane surface.
Generally, there are three basic issues which are addressed in the prior art concerning hydrogen purifiers: 1) membrane alloy selection, 2) mechanical support of the membrane, and 3) sealing of the membrane in a purification structure. In some cases the mechanical support and the sealing means are interrelated.
For example, in U.S. Pat. No. 6,183,542 Bossard discloses a foil-based hydrogen purifier where a hydrogen permeable foil, such as PdAg, is bonded between two wire mesh structures. The bonding process is envisioned either as a brazing step (by coating the mesh with a brazing powder) or by joining the materials at high temperature under pressure in a vacuum furnace in the range of 1900° F. (1038° C.). The resulting membrane does not lay flat, but rather undulates between the various wires of the mesh. In order to seal the perimeter, Bossard suggests utilizing a peripheral brazed seal, which would presumably fill the open areas of the metal mesh. At these high temperatures, however, significant interdiffusion between the support and the membrane is expected to lower the hydrogen permeability.
Ogawa et. al show a composite structure for hydrogen purification in U.S. Pat. No. 5,782,960. Here the inventors utilize a foil bonded or laminated to a porous metal member. In the preferred embodiment the invention utilizes plural metallic supports with rectangular openings formed by an etching process. In the invention the supports are etched prior to attachment of the membrane. The described means of bonding or laminating consists of diffusion bonding or brazing.
In US 2003/0033933, Frost and B. Krueger further explore the patenting process with the membrane separator of Allegheny Technologies. In this and in preceding patents a hydrogen-permeable foil is disposed over a disc with a seal at the center and periphery (formed by welding), where the mechanical support of the membrane is accomplished with the use of a metal mesh. In this particular patent application Frost and Krueger further add a coating over the wire mesh of a nitride, oxide, boride, silicide, carbide, or aluminide to prevent intermetallic diffusion between the mesh and the membrane. The interdiffusion of iron and other elements is known to reduce the hydrogen permeability of the palladium alloy membrane, which as claimed by the inventors is blocked by the coatings. Similar architectures are also disclosed in U.S. Pat. No. 6,602,325, U.S. Pat. Nos. 6,835,232, and 6,582,499.
Another approach is taken by Juda et. al in U.S. Pat. No. 5,904,754. Here a method is disclosed for forming an effective perimeter seal with a PdCu membrane, utilizing diffusion bonding. Specifically, the inventors utilize a copper-surfaced metallic frame, compress the PdCu membrane to the perimeter, and condition the assembly in a hydrogen atmosphere at 290-325° C. for several hours to form an effective sealing diffusion bond between the frame and the membrane.
Li uses a similar approach in US 2005/0109821. However the inventor claims an improvement by utilizing a PdAg membrane (which can be processed at higher temperatures than PdCu), and eliminating the copper plating at the frame. Li observed strong bonding between the unplated stainless steel frame and the PdAg foil by diffusion bonding for 30 hours in a hydrogen atmosphere at 650° C.
While all of the above examples allow for the fabrication of membrane purifiers, each has certain inherent limitations. For example, with planar structures utilizing diffusion brazing or bonding to create perimeter seals, the surfaces must be highly polished or very flat in order to affect a seal. Small surface imperfections in the form of high or low spots, or scratches, which are typically present, make it very difficult to form a hermetic seal.
Brazing allows for some imperfections to be present while still yielding a seal, since the braze material will reflow and fill crevices with capillary action. However, brazing has two drawbacks. First, the high processing temperatures required to melt the braze can cause intermetallic diffusion to occur between a metallic membrane and the membrane support, as well as alteration of the crystal structure of the membrane. For example, processing Pd60Cu40 membranes above 400° C. for any length of time can cause permanent phase separation of the alloy, forming copper-rich regions, which destroys the hydrogen permeability of the material. Second, there is the risk that the brazing materials may contact the membrane, forming unwanted alloys while consuming the membrane. In some cases the braze material in contact with the membrane will actually cause perforations in the membrane.
Welding can be used to form a perimeter seal, however the material proximal to the weld may come under significant stress when the membrane expands on hydrogen uptake, while the other material (such as stainless steel) does not expand. This stress may cause eventual rupture and leakage of the seals after cycling the purifier thermally or with hydrogen a number of times. It is also difficult to form a welded seal on materials as thin as a 25 micron membrane, when the other materials (i.e., stainless steel parts, etc.) are much thicker.
Therefore an improved architecture and method of forming a hermetic seal with the purifier is needed.